For the purposes of conciseness, in the subsequent description, the temperature and pressure conditions under which one and/or the other of two fluid phases present is in the supercritical state will be referred to by the generic term “supercritical conditions”. Thus, two fluid phases termed “in supercritical conditions” or “under supercritical conditions” in the present description comprise (1) a first phase in the supercritical state and (2) a second phase, in contact with the first, and where said second phase is in the liquid, gaseous or supercritical state (generally liquid or gaseous). Two fluid phases termed “under supercritical conditions” according to the present description are therefore not necessarily both in the supercritical state. Stated otherwise, two phases having critical temperatures of T1 and T2 and critical pressures P1 and P2 respectively, the phases will be considered to be “under supercritical conditions” within the meaning of the present description if, and only if:                the temperature is greater than T1 and the pressure is greater than P1; and/or        the temperature is greater than T2 and the pressure is greater than P2.        
The aforementioned interfacial tension under supercritical conditions exists between two non-miscible fluid phases under the supercritical conditions of measurement. Unless explicitly specified to the contrary, the expression “non-miscible fluid phases” refers, in the present description, to two phases in the liquid, gaseous or supercritical state and which are not miscible under the conditions of implementation of the method (it being understood that the two phases could optionally be miscible under other conditions).
Access to the knowledge of the value of interfacial tensions between two fluid phases under supercritical conditions is of importance, in numerous technological sectors. This parameter may indeed turn out to be critical in particular in certain physico/chemical methods employing a phase in the supercritical state, or else liable to lead in the course of their implementation to supercritical conditions. Inter alia, access to the value of the interfacial tension is of interest for processes employing CO2 in the supercritical state, which can be used for example in syntheses or methods not employing any organic solvents; in petroleum recovery methods; or else for the capture and storage of CO2.
The determination of an interfacial tension between two fluid phases under supercritical conditions is known as being relatively complex to implement. In fact, it generally requires heavyweight apparatus, in particular having regard to the high pressures which are employed. Moreover the procedures which have been proposed to date for the measurement of interfacial tensions in a supercritical medium generally involve long durations of measurement, as well as relatively significant volumes, with associated risks for the operators (supercritical conditions involving risks of explosion or of leakages which increase with the duration and the quantities). In addition to these safety problems, the proposed methods are often limited to the analysis of certain specific fluids and the conditions of analysis have to be adapted for each fluid pair studied.
The scant procedures which have currently been proposed for the measurement of interfacial tensions under supercritical conditions, prone to the aforementioned drawbacks, typically implement high-pressure visualization cells, within which the interfacial tension is determined according to the so-called “pending drop” (or “hanging drop”) technique, where the measurement is performed by analyzing the shape adopted by a drop of a dense phase suspended within a less dense phase. For further details in this regard, reference may in particular be made to U.S. Pat. No. 5,653,250 or else to the article by Adkins et al. In the Journal of Colloid and Interface Science, vol. 346, p. 455 (2010).